Estrogen synthetase is an enzyme which catalyzes the final step in the biosynthesis of estrogens. We will purify the cytochrome P-450 protein from mammalian ovaries and characterize it biochemically in reconstituted systems. A knowledge of the properties of this enzyme, whose normal function is essential to reproduction, may have important practical implications in the fields of reproductive endocrinology, fertility regulation and carcinogenesis in the female reproductive tract, as well as providing further insight into the basic properties of these heme containing enzymes.